In separate-type apparatuses (for example, air conditioning systems) in which an outdoor unit and an indoor unit are separated, in general the outdoor unit and the indoor unit are connected by the three connection lines of a power source line, a communication line and a common line (a line common for both power transmission and communication). The outdoor unit and the indoor unit receive a supply of power from an alternating current power source connected between the power source line and the common line.
In addition, the outdoor unit comprises a communication circuit in which a direct current power source that supplies several dozen volts of direct current voltage, a switching element and a receiving element are connected in series, and the indoor unit comprises a communication circuit in which a switching element and a receiving element are connected in series. The communication circuit installed in the outdoor unit and the communication circuit installed in the indoor unit form a closed circuit by being connected in parallel between the communication line and the common line. Furthermore, the outdoor unit and the indoor unit are able to control the loop current flowing in the closed circuit and communicate in both directions by switching the switching elements on and off.
The connection between the outdoor unit and the indoor unit is normally accomplished at the location where the outdoor unit and the indoor unit are located. Hence, erroneous connection of the connection line to the outdoor unit or the indoor unit can occur. When the alternating current power source is erroneously connected to both terminals (the input terminal and the output terminal) of the communication circuit operating at a lower voltage than the alternating current voltage supplied form the alternating current power source, the communication circuit can be destroyed by insufficient ability to withstand voltage (impressing of excess voltage) of the communication circuits of the outdoor unit and the indoor unit, or by excess current flowing in the outdoor unit or the indoor unit. Consequently, it is necessary to have, in the outdoor unit and the indoor unit, a protection means that protects the communication circuit from excess current or excess voltage.
An indoor air conditioning device utilizing a thermistor having a positive temperature coefficient is disclosed in Patent Literature 1 as a protection means. When an excess current flows in the communication line of this indoor air conditioning device, the excess current flows through the thermistor. Accompanying this, the thermistor generates heat and the resistance component of the thermistor increases, limiting the excess current.
In addition, a communication control device in which a current limiting circuit comprising a current-limiting bipolar transistor, a bipolar transistor for regulating base-emitter voltage and a current detection resistance is connected in series with a communication circuit is disclosed in Patent Literature 2. In this communication control device, the series circuit made up of the emitter-collector path of the current-limiting bipolar transistor maintained in a saturated state and the current detection resistance is connected to a communication circuit. The voltage (end-to-end voltage) of the current detection resistance increases accompanying increases in the electric current flowing in the communication circuit. When the voltage of the current detection resistance exceeds a prescribed voltage, the bipolar transistor for regulating base-emitter voltage regulates the base-emitter voltage of the current-limiting bipolar transistor and acts to keep the voltage (end-to-end voltage) of the current detection resistance constant. Because the upper limit of the current flowing in the communication circuit is limited by this action, the current limiting circuit can protect the communication circuit from eddy current.